1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal by Czochralski method (CZ method) comprising using a silicon seed crystal (hereinafter occasionally referred to as seed crystal), performing a necking operation, and growing a silicon single crystal ingot.
2. Description of the Related Art
In a conventional method of manufacturing a silicon single crystal according to CZ method, a seed crystal which is a silicon single crystal is brought into contact with silicon melt and is then slowly pulled while being rotated to grow a silicon single crystal ingot. In the method, in order to eliminate dislocation caused by propagation of slip dislocation generated in the seed crystal in high density due to thermal shock, a neck portion having a smaller diameter as approximately 3 mm is formed after the silicon seed crystal is brought into contact with a silicon melt, namely so called necking operation is performed. Subsequently, the diameter of the crystal is increased to be a predetermined value, and then a dislocation-free silicon single crystal can be pulled. The above-mentioned necking operation is well known as Dash Necking method, which is a common method for pulling a silicon single crystal ingot according to CZ method.
The seed crystal conventionally used is in a cylindrical shape having a diameter of approximately 8 to 20 mm or a prismatic shape having a side length of approximately 8 to 20 mm, wherein a cut-away portion or notch is formed thereon for attachment to a seed crystal holder, and a shape of a lower end thereof to be brought into contact with a silicon melt first is flat. To pull a heavy single crystal ingot safely, thickness of the seed crystal cannot be smaller than the above value in light of strength of the material.
In the seed crystal having such a shape, slip dislocation generates in high density, since a heat capacity of the lower end which is brought into contact with a silicon melt is large, a temperature difference generates rapidly in the crystal as soon as the seed crystal is brought into contact with the silicon melt, resulting in generation of dislocation in high density. Accordingly, the above-mentioned necking operation is necessary to eliminate slip dislocation in the single crystal.
However, in the above mentioned method, a minimum diameter of the neck portion has to be decreased to approximately 3 to 5 mm in order to eliminate the dislocation, even when the necking condition is chosen appropriately. Such a small diameter is insufficient in strength to support a single crystal ingot such as manufactured in recent years, which has been getting heavier with increase of a diameter thereof. This may lead to a serious accident such that the fine neck portion is broken while the single crystal ingot is pulled, and the single crystal ingot falls.
To solve the above-mentioned problems, the applicant proposed inventions as disclosed in Japanese Patent Application Laid-open (kokai) No. 5-139880 and No.9-255485 (Japanese Patent Application No. 8-87187). In these inventions, a seed crystal having a wedge or hollow lower end is used to reduce as much slip dislocation which is generated when the seed crystal is brought into contact with a silicon melt as possible, so that dislocation can be eliminated even when the neck portion is relatively thick, and thereby the strength of the neck portion can be improved.
According to the method, strength of the neck portion can be improved to some extent, since the neck portion can be formed to be thick. However, even in the method, a necking operation is performed and a neck portion in which slip dislocation is present is formed. Furthermore, the neck portion has to be thicker for manufacture of an ingot which is larger in a diameter and longer such as those manufactured in recent years. For example, the diameter of the neck portion has to be 5 mm at least in order to pull a single crystal ingot having a weight of 200 kg or more, otherwise the strength may be insufficient. Accordingly, these inventions cannot solve the problems fundamentally.
Another problem in the necking method using the seed crystal having the special shape of the tip end mentioned above relates to a rate of success in making a crystal dislocation free. When the elimination of dislocation results in failure in the above-mentioned method, the seed crystal has to be exchanged to perform the method again. Accordingly, improvement in the rate of success in making a crystal dislocation free is especially important in the method. Elimination of dislocation cannot be achieved with a thick neck. According to a conventional necking method, when a diameter of a neck is more than 6-7 mm, elimination of dislocation is hardly achieved.
The inventors studied the cause of lowering of the rate of success in making a crystal dislocation free, and found that control of the factors which have been controlled in the conventional methods, such as a shape of a seed crystal, a temperature maintaining time during which a seed crystal is held above a melt surface, a melting speed, or the like is not sufficient for improvement in the rate of success in making a crystal dislocation free and reproducibility.